I think NIce Guy is very right , there is no direct calculation for converting Data Traffic to voice traffic or vise versa.
all This calculation found in Internet is for Voice over IP or simple term when channels/TC are very fixed ie 1 or 2 ch/TC, may ch having capacity to deliver 64 Kbps, can calculate the erlang to Kbps.
but when throughput is depend on, dynamically used Time slot and depend on request of Terminal